The new Ford Fiesta has slowly gained converts among Americans, hitting a high in sales last month. But Ford's now facing a growing number of complaints about the Fiesta's new dual-clutch automatic transmission set to go into other Ford models as well.

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None of the complaints suggest a safety problem which would force Ford to do a recall, and Ford has been agressive about responding to the complaints on Twitter and elsewhere. But the reports suggest a real issue with the Fiesta, one that could dent Ford's recently burnished quality reputation.

One of the Fiesta's calling cards is fuel economy of up to 40 mpg on the highway, made possible by the Powershift six-speed automatic transmission built at a Ford-Gertrag joint venture in Mexico. The all-new Powershift is essentially two three-speed manual transmissions merged and shifted electrically with two clutches. Using electrical controls rather than traditional hydraulics helps the transmission boost the fuel economy of the Fiesta by 10%, and Ford will use it on the upcoming Focus as well.

Yet since its launch last year, a sizable number of consumers have reported problems with the Powershift, both online and to U.S. auto safety regulators, ranging from hard shifts at speed to not engaging any gear. Several have reported having to get new transmissions, and there's at least two reports of multiple transmission failures.

We had the same issue with our 2011 Ford Fiesta…a few times it started rough..clicked three times then started..It happened in the cold weather then my wife backed out of the drive way one day and it wouldn't go into drive. We had it towed to the dealer and they said when they got it off the flat bed they got in and drove it into the garage no problem. They ran a bunch of tests on the system and couldn't get the car to do it again.

Five owners have filed complaints with the National Highway Traffic Safety Administration over the Powershift, with two reporting they were given replacement units. Other owners on the Ford Fiesta forum say they're having to wait for replacements, with their Fiesta sitting in a dealer's lot:

Owned it for 10 days of short distance driving. Took it back to the dealer who said they would have to change out the complete transmission assembly on order from Ford. Big problem is they had no spares anywhere in the US of A! Swear to God - that's what Ford told me! So the car has been at the dealers for over two weeks because they said the problem was too serious to continue driving the car.

Ford has issued one repair bulletin for the Fiesta's transmission to dealers, saying low voltage in the car's electrical system could cause the transmission's electronic controls to shut down or not allow the car to start at all, as this video shows. The fix? A software update for the transmission along with scraping paint away from underneath the battery's ground screw for a better connection.

Many owners say that fix solved their problem, but others are still in line for brand-new transmissions.

Ford says only a small number of transmissions have been replaced, in part to diagnose what was causing them to fail, and that the low-voltage issue was an early production issue that's since been fixed. That could explain it, although several of the complaints we found were made last month. American automakers don't get the benefit of the doubt from consumers on quality, and given how much Ford has riding on the Fiesta and the new Powershift, its fixes need to work.